Ensure multi-item chunks in couch_btree:chunkify/1
Ensure multi-item chunks in couch_btree:chunkify/1

If the last element of a chunk has a huge reduction it was possible to
return a btree node that had a single key. This prevents the edge case
by forcing it into the previous chunk. Without this we can end up with a
case where a path in the tree can extend for many levels with only a
single key in each node.

COUCHDB-3298

apache/couchdb-couch,apache/couchdb-couch,cloudant/couchdb-couch,fkaempfer/couchdb,cloudant/couchdb-couch,fkaempfer/couchdb,cloudant/couchdb-couch,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,apache/couchdb-couch